Digital display systems using cathode ray tube display devices have been used for many years. The earliest of these devices used a beam positioning arrangement in which the digital input signals defined the deflection of the C.R.T. beam so that lines were drawn on the C.R.T. face as the beam moved along the path defining each line. Such systems have now been largely replaced by raster scanning systems in which the display is generated by modulating the CRT beam as it scans across the CRT face in a repetitive raster configuration. There are two arrangements for generating the beam modulation signal, the first is the character generation method, and the second is by use of an all points addressable refresh buffer store. It is to the second of these that the present invention relates. In this arrangement, digital data groups representing picture elements of the display are stored in sequence in a large refresh buffer store. They are stored in the same sequence as they are required to generate the picture elements on the screen. In order to refresh the C.R.T. display, the groups are read out in sequence to drive the display.
An early example of an all points addressable display arrangement is shown in U.S. Pat. No. 3,293,614 (Fenimore et al). In one embodiment of the system shown therein, the refresh store has one bit position for each screen picture element. These bits are read from the store in an order and at a rate corresponding to their presentation on a cathode ray tube display. In this system, each displayed picture element is represented by a single bit only so that the CRT beam is only either on or off for each position, and no colors or graduated gray elements can be displayed. Even with this limitation, for the described display, which has 512 elements per line and 410 useful lines per a frame, total of 209,920 stored data bits are required. For a later described embodiment using a color display, four stored bits for each picture element are used, this requires a storage space of about 840,000 bits or about 105K bytes.
Thus, the all points addressable system is relatively expensive in terms of buffer storage requirements. On the other hand, there is a requirement, especially in the color graphics display field, for more bits per picture element to define more different colors on the display. Similarly for some black and white displays high definition half tone images are required.
In order to increase the number of available colors or gray tones in a display, the palette system was developed. An early example of such a system is shown in an article entitled `Computer Graphics in Color` by P. B. Denes, which appeared in the Bell Lab. Records, Volume 52, May, 1976 on pages 139 through 146. In that system, points addressable refresh buffer memory is arranged to provide three bits for each picture element to be displayed. This, of course, would normally provide data to permit 8 different colors on the display tube. However, instead of driving the color drive signals directly from the refresh buffer data, each data group of 3 bits is used to select one of 8 sets of palette registers. Each of these sets stores a total of 21 bits of data, of which repetitive groups of 7 are used to generate red, blue and green signals through repetitive digital-to-analog converters. The feature which gives this color palette system great color flexibility is that the contents of the registers can be altered by the computer driving the display system. The article states that typically they might be changed after the display of each frame is completed. The major limitation of the system is that frequent changes of the data in the registers, that is, several changes within a display frame time, uses an undesirable amount of computer time. Thus, to achieve efficient operation of the computer, each display frame is normally restricted to eight colors, though these colors can be changed for successive frames.
One method of overcoming this limitation, though in a very restricted application, is shown in U.S. Pat. No. 4,225,861 (Langdon et al). In that arrangement, the palette system, called "video lookup table" in the specification, has four zones. The palette system is addressed by the picture element outputs from the refresh buffer together with two selected bits of each address of the refresh buffer used to read out this buffer. These selected bits direct successive picture element outputs to successive ones of the zones. By this means, a textured display pattern is generated.